A review focusing on mechanisms and ecological risks of enrichment and propagation of antibiotic resistance genes and mobile genetic elements by microplastic biofilms.

Microplastics (MPs) are emerging ubiquitous pollutants in aquatic environment and have received extensive global attention. In addition to the traditional studies related to the toxicity of MPs and their carrier effects, their unique surface-induced biofilm formation also increases the ecotoxicity potential of MPs from multiple perspectives. In this review, the ecological risks of MPs biofilms were summarized and assessed in detail from several aspects, including the formation and factors affecting the development of MPs biofilms, the selective enrichment and propagation mechanisms of current pollution status of antibiotic resistance genes (ARGs) and mobile genetic elements (MGEs) in MPs biofilms, the dominant bacterial communities in MPs biofilms, as well as the potential risks of ARGs and MGEs transferring from MPs biofilms to aquatic organisms. On this basis, this paper also put forward the inadequacy and prospects of the current research and revealed that the MGEs-mediated ARG propagation on MPs under actual environmental conditions and the ecological risk of the transmission of ARGs and MGEs to aquatic organisms and human beings are hot spots for future research. Relevant research from the perspective of MPs biofilm should be carried out as soon as possible to provide support for the ecological pollution prevention and control of MPs.

Fine particles and pyrogenic carbon fractions regulate PAH partitioning and burial in a eutrophic shallow lake.

Aquatic particles and organic carbon (OC) regulate the occurrence and transport of hydrophobic organic contaminants such as polycyclic aromatic hydrocarbons (PAHs) in water-suspended particle-sediment interfaces. Conventional studies on the mechanisms regulating the relationships between PAHs and total particles/OC have ignored micro-scale regulatory factors such as particle size and OC composition. Field research in the eutrophic shallow Lake Taihu, China, revealed that the fine particle fractions 2.7-10 µm in diameter had stronger PAH adsorption capacity and significantly regulated PAH particle size distribution and water-particle partitioning. Selective PAH biodegradation by planktonic microorganisms probably significantly weakened the capacity of the coarse fractions to regulate PAHs. OC fragments at different temperature gradients had markedly different influences on the particle size distribution of PAHs. High-temperature pyrogenic OC fractions (part of black carbon) were the principal OC regulatory factors for medium-to high-molecular-weight PAHs. However, the OC fragments did not directly affect the particle distribution of low-molecular-weight PAHs. During particle deposition and burial, microbial PAH utilization and efficiency probably regulated the burial potential of various hydrophobic PAH species. Biodegradation of relatively less hydrophobic PAHs with octanol-water partition coefficients (log Kow) < 5.8 showed an increasing trend with decreasing PAH hydrophobicity. Biological pump action of the relatively higher hydrophobic PAH species (log Kow > 5.8) showed a decreasing trend with increasing PAH hydrophobicity. The discoveries of the present work further clarified the mechanisms of PAH partitioning and burial in a eutrophic shallow lake and collectively provides a valuable reference for modeling the transport and dispersal mechanisms of hydrophobic, particle-bound organic contaminants in other aquatic ecosystems.

Shifts in the relative abundance and potential rates of sediment ammonia-oxidizing archaea and bacteria along environmental gradients of an urban river-estuary-adjacent sea continuum.

Ammonia-oxidizing archaea (AOA) and bacteria (AOB) play important roles in N cycling in sediments globally. However, little is known about their ammonia oxidation rates along a river-estuary-sea continuum. In this study, we investigated how the potential ammonia oxidation rates (PARs) of AOA and AOB changed spatially along a continuum comprising three habitats: the Shanghai urban river network, the Yangtze Estuary, and the adjacent East China Sea, in summer and winter. The AOA and AOB PARs (0.53 ± 0.49 and 0.72 ± 0.69 µg N g-1 d-1, mean ± SD, respectively) and their amoA gene abundance (0.47 ± 0.85 × 106 and 2.4 ± 3.54 × 106 copies g-1, respectively) decreased along the continuum, particularly from the urban river to the estuary, driven by decreasing sediment total organic C and N and other correlated inorganic nutrients (e.g., NH4+) along the gradient of anthropogenic influences. These spatial patterns were consistent between the seasons. The urban river network, where the anthropogenic influences were strongest, saw the largest seasonal differences, as both AOA and AOB had higher PARs and abundance in summer than in winter. The ratios between AOA and AOB PARs (~0.87 ± 0.51) and gene abundances (~0.25 ± 0.24), however, were predominantly <1, indicating an AOB-dominated community. Comparing the different NH4+ consumption pathways, total aerobic oxidation accounted for 12-26% of the total consumption, with the largest proportion in the estuary, where the system was well oxygenated, and the lowest in the adjacent sea, where inorganic N was highly depleted. This study revealed the spatiotemporal patterns of AOA and AOB potential rates and gene abundance along gradients of human influences and identified organic matter and nutrients as key environmental factors that shaped the variation of AOA and AOB along the continuum.

Sediment nitrate reduction processes in response to environmental gradients along an urban river-estuary-sea continuum.

Sediment denitrification (DEN), anaerobic ammonium oxidation (Anammox), and dissimilatory nitrate reduction to ammonium (DNRA) are three important nitrate (NO3-) reduction pathways in aquatic ecosystems. These processes modify nitrogen (N) loadings from land to the ocean, with important implications on the management of coastal eutrophication. While NO3- reduction has been studied intensively for various types of habitats, studies on its distributions along river-estuary-sea continua remain scarce. In this study, we examined these three pathways along a N-laden urban river-estuary-sea continuum comprised of three types of habitats (urban river, estuary, and adjacent sea) in the densely populated Shanghai-East China Sea area. The potential DEN, Anammox, and DNRA rates decreased seaward both in summer and winter in response to decreasing sediment organic matter (OM, 20 to 7 to 7 mg C g-1), ferrous oxide (9 to 2.7 to 2.8 mg Fe g-1), and bottom water dissolved inorganic nitrogen (543 to 112 to 21 µM). Among these pathways, DEN remained a major component (~69.6%) across habitats, while Anammox (47.9%) rivaled DEN (48.3%) in the urban river in winter. N retention index (NIRI), the ratio between retained and removed NO3-, ranged from 0 to 0.5 and increased downstream. Together, these results suggest that the decreasing gradients of OM and inorganic matter shape the distribution of NO3- reduction along the continuum, reflecting the diminishing impact of the river and human inputs from the urban river to the ocean. Our results highlight the importance of taking a continuum perspective in N cycling studies and emphasize the role of urban rivers as N removal hotspots, which should be a focus of research and management.